Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, and potentially compressor surge. Surge can lead to NVH issues such as undesirable noise from the engine intake system.
Transient response of boosted engines can be improved by adding electric assist to the turbocharger. This, however, increases the propensity for compressor surge, especially during tip-in maneuvers that require rapid increase in boost pressure. Traditional methods for surge control completely open a compressor recirculation valve responsive to surge so as to dump boost pressure from downstream of the compressor to upstream of the compressor and are not suitable for this application.
One example approach to address surge in engine systems configured with electrically assisted turbochargers is shown by Barthelet et al in U.S. Pat. No. 7,779,634. Therein, responsive to an indication of surge, a controller coordinates the operation of a valve in a path coupling an air intake passage to an exhaust passage with the operation of an electric assist motor of a boosted engine. The coordinated operation relies on turbocharger speed information to increase a compressor map width (that is, a compressor margin to surge) by increasing air flow from the air intake passage to the exhaust passage while controlling the electric motor to maintain or increase a compressor rotational speed.
However, the inventors herein have recognized potential issues with such approaches. As one example, introducing fresh air into the exhaust system may cause an exhaust three-way catalyst to become oxygen loaded, and thereby unable to properly treat regulated exhaust constituents unless additional fuel is injected to burn with the air in the exhaust system. Doing so increases exhaust system temperatures which must be limited to avoid damaging the turbocharger and catalyst. The additional fuel also reduces engine fuel economy. As another example, there may be conditions where there is not enough positive pressure differential from intake to exhaust to produce sufficient flow to prevent compressor surge. Depending on intake and exhaust system design, the pressure differential may be negative at the conditions of interest. In this scenario, flow would recirculate from the exhaust to the intake with the valve open (i.e., exhaust gas recirculation or EGR). The resulting EGR flow which may be beneficial for emissions or fuel economy but would not reduce surge margin. As another example, the method relies on turbocharger speed which may not be measured due to cost and/or durability or accurately inferred.
In one example, some of the above issues may be addressed by a method for a boosted engine, comprising: responsive to an indication of surge following a tip-in or tip-out event, increasing a margin to surge while maintaining boost pressure at a level based on torque demand via adjustments to each of an output of an electric motor coupled to a boosting device and an opening of a continuously variable compressor recirculation valve (CCRV), the adjustments selected based on the torque demand following the tip-in or tip-out event. In this way, distinct airflow adjustments may be provided to address surge that is responsive to an increase in torque demand versus surge that is responsive to a decrease in torque demand, improving boosted engine performance and responsiveness.
As one example, a boosted engine may be configured with a turbocharger. Responsive to an indication of surge, such as tip-in surge, an engine controller may increase the opening of a continuously variable compressor recirculation valve (CCRV) coupled across an intake compressor of the turbocharger to increase the margin to surge. At the same time, a waste-gate opening may be decreased based on the opening of the CCRV so as to maintain the operator demanded boost pressure and balance the shaft power. Further, the controller may increase a power output by an electric motor coupled to the turbocharger, such as to a shaft of the turbocharger (also referred to herein as an electric assist provided by an electric assist motor) to provide the required increase in airflow to improve the margin to surge while also maintaining the desired boost pressure and balancing shaft power.
Further, in response to tip-out surge where the tip-out is moderate and driver torque demand requires the engine to continue operating under a boosted condition, the controller may similarly increase margin to surge while concurrently maintaining boost pressure. In response to tip-out surge where the tip-out results in driver demand torque that can be met without boost pressure, the controller may increase margin to surge by opening a compressor recirculation valve coupled across an intake compressor without supplying electric power to the electric motor coupled to the turbocharger shaft or by extracting power from the electric motor, thereby slowing the turbocharger shaft.
It will be appreciated that while the above example is described with reference to an engine system where the boosting device is a turbocharger, in alternate examples, the boosting device may be an electric supercharger and the adjustments may be performed via an electric motor coupled to the supercharger compressor.
In this way, the operation of a compressor recirculation valve may be coordinated with the amount of electric motor torque provided to a turbocharger shaft to improve surge control. The technical effect of increasing the opening of the CCRV while increasing the output of the electric assist motor is that a more aggressive control of the electric assist motor is possible for a faster boost response. This improved boost response may be useful when the surge occurs during a tip-in event where operator demanded peak torque increases. Further, a robust approach for surge mitigation is provided while enabling the desired airflow to also be delivered. By coordinating the operation of the electric motor with a CCRV to optimize engine performance in the presence of a surge constraint, the same motor can be used for improving a margin to surge during a tip-in event by adding torque to a boosting device (e.g., a turbocharger shaft) while improving the margin to surge during a tip-out event by subtracting torque from the boosting device (e.g., the turbocharger shaft). Overall, boosted engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.